有效地确定JSON标签

Question

I have a bunch of JSON files, each containing a very large array of complex data. The JSON files look something like:

ids.json

{
    "ids": [1,2,3]
}

names.json:

{
    "names": ["Tyrion","Jaime","Cersei"]
}

and so on. (In reality, the array elements are complex struct objects with 10s of fields)

I want to extract just the tag that specifies what kind of array it contains. Currently I'm using encoding/json to unmarshal the whole file into a map[string]interface{} and iterate through the map but that is too costly an operation.

Is there a faster way of doing this, preferably without the involvement of unmarshaling entire data?

Answer 1

You can offset the reader right after the opening curly brace then use json.Decoder to decode only the first token from the reader

Something along these lines

sr := strings.NewReader(`{
    "ids": [1,2,3]
}`)

for {
    b, err := sr.ReadByte()
    if err != nil {
        fmt.Println(err)
        return
    }
    if b == '{' {
        break
    }
}

d := json.NewDecoder(sr)

var key string
err := d.Decode(&key)
if err != nil {
    fmt.Println(err)
    return
}

fmt.Println(key)

https://play.golang.org/p/xJJEqj0tFk9

Additionally you may wrap your io.Reader you obtained from open with bufio.Reader to avoid multiple single-byte writes

This solution assumes contents is a valid JSON object. Not that you could avoid that anyway.